The demand for higher performanced carbon fibers is becoming stronger recently. Particularly in the field of aircraft, there is strongly required high elongation and high modulus carbon fibers. Such requirement has been directed more to enhancement of composite properties rather than to the carbon fiber itself.
Under these situations of requirement of the market, the tensile strength of PAN-based high modulus carbon fibers having 30 to 50 ton/mm.sup.2 modulus has been markedly improved to be 300 kg/mm.sup.2 or more based on the improvement in precursors and optimization of carbonization techniques.
The tensile strength of carbon fiber is generally determined, as described in JIS-R-7601, with specimens in the state of a strand, one of the forms of composite materials. Studies made by the present inventors have revealed that the tensile strength of carbon fibers in the strand test is markedly influenced by the characteristic of matrix resins and the interfacial bond strength. The matrix resin used in aircraft is usually a thermosetting resin of heat-resistant type called "350.degree. F.-type", and is a brittle and low elongation type owing to its high cure-molding temperature and high crosslinking density. The present inventors have now reached the conclusion that the tensile strength of the composite decreases with increasing brittleness of the matrix resin as in 350.degree. F. type resin and the tendency becomes stronger with increase of the tensile strength of the carbon fiber. More importantly, the tensile strength of the composite of 350.degree. F. type resin reinforced with the high strength and high modulus carbon fiber decreases as the interfacial bond strength increases.
Conceivable reasons for this include the following. First, since residual strain due to cureshrinkage is large in 350.degree. F.-type matrix resins, residual stress tends to be more concentrated in the neighborhood of the fiber with increasing interfacial bond strength. Secondly, 350.degree. F.-type matrix resin is a so brittle type that the cohesive failure of the resin phase tends to proceed from the initial breaking point of the fiber by notch effect when the interfacial bond strength is high, which results the breakages of sub-bundles of 50 to 100 filaments leading to the whole breakage of the composite; moreover, such a tendency becomes stronger with increasing strength of carbon fibers because of the high fracture energy. In any way, it has been revealed that the tensile strength of composites of a high strength carbon fiber and a 350.degree. F.-type brittle matrix resin is rather decreased when the interfacial bond strength is too high.
Further, it has been revealed that the flexural strength of composites sometimes shows, much lower value than that expected from the rule of mixture, unlike the flexural modulus, namely the translation rate of fiber strength is sometimes markedly low, and moreover the translation rate tends to decrease with the increase of tensile strength. The reason for this is considered to be that the flexural strength of composite material is greatly influenced by the interfacial bond strength between the fiber and the matrix resin as well as failure behaviour, and the influence becomes stronger as the fiber strength is more improved and resultantly the fracture energy is increased.
It has been revealed that the flexural strength of composites is strongly dependent on the level of surface treatment, decreasing both when the interfacial bonding strength is too high and when it is too low, and thus there exists an optimum interfacial bond strength, namely an optimum surface state, for the flexural strength.
Accordingly, the optimum surface state for tensile strength is not always optimum for flexural strength of composite materials, which can result in the decrease of translation properties in flexural strength and renders it difficult to make the most of the improved strength of the reinforcement.
Further, since the optimum surface state varies greatly depending on the difference of starting fibers or heat-treatment conditions even when the surface-treatment conditions are the same, it is necessary for the surface treatment to be properly performed in correspondence with the above variations.
The present inventors made extensive studies about surface characteristic of the high performanced carbon fiber to give the optimum interfacial bond strength for balancing the tensile strength against the flexural strength of the composite material, and as a result attained this invention.